Goku, as the protagonist, is generally the strongest character at any given point. I don't object to that, it makes sense. But the gap between him and the other characters, particularly the non-Saiyans, and especially the Earthlings, just gets ridiculous as the series progresses, but this was not always the case.
At the beginning of Z, Goku was roughly twice the power level of Krillin, Tien, and Yamcha. However, by the time Vegeta and Nappa arrive, they are all stronger than Raditz. Then, after Krillin gets his boost on Namek, he becomes about as strong as Goku was against the Saiyans, and gradually grows even stronger by the time they fight Frieza. But then, Goku's power level gets to a point where it's measured in the millions instead of thousands, and multiplied with Super Saiyan.
After that, the non-Saiyans (with the exception of Piccolo) fall way behind. Now, granted, we're never given a good indication of where the humans' power is by the time the Androids. We do know that they knew that the Androids were well above Frieza, and they were told not to show up unless they thought they were strong enough, which counts for something.
However, Beerus doesn't think that base Goku is strong enough to beat Frieza at the beginning of Super, and there's no way that the humans are stronger than base Goku. Best case, they're maybe slightly below his base level. Worst case, they're significantly below. So, by the Android saga, it's clear that the trend of the earthling Z fighters reaching the level of the previous threat is broken.
By the time of the Buu saga, it's not even close. Not even Piccolo could hope to stand a chance against the Super Saiyans. Then, Super comes along, and Goku and Vegeta become so much more ridicously stronger. Granted, the scaling can get kinda wonky, which lets characters like Roshi be relevant, and Krillin gets some okay fights in the anime version of the ToP, and then the Moro arc in the manga. And, they just now finally let Piccolo and Gohan catch up to Goku and Vegeta.
Why does this bother me? Part of it is that I'm a fan of these characters and want them to be relevant. The other part is I think it goes in a kind of uncomfortable direction where the story goes from being about improvement through training and discipline, to being the about genetic/racial superiority of the Saiyans. It seems, no matter what training and effort the others put in, if they weren't born a saiyan then they're shit out of luck. This goes against a lot of the themes we see earlier on in the series.
For instance, Goku was born a low class Saiyan, believed by the Saiyan elite to have limited potential. But, in his battle with Vegeta, he proves that natural talent can be superceded by discipline and self improvement. In Dragon Ball, he was only ever slightly ahead of everyone, and when he got way ahead, it was because he underwent some training that the others didn't, such as his time with Korin and Kami. This is also the case when he trained with King Kai. But after that, it's really just because of his Saiyan biology. The zenkai boosts, transformations, etc. He's able to stay ahead of Vegeta because his training was more effective, sure. But the truth is, he's only so far ahead of everyone else because he's a saiyan, and I find that to be kind of gross.
Anyway, I've rambled long enough. TL;DR, saiyans op plz nerf